Sediment mobility modelling and hydrodynamic properties of maerl

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Sediment mobility modelling is a useful tool for scientifically robust marine spatial planning and characterisation of the benthic disturbance regime. As a prerequisite to modelling sediment transport of free living coralline algae habitats known as maerl or rhodolith, it is necessary to know fundamental hydrodynamic properties of this biogenic sediment. Two hydrodynamic properties, settling velocity and critical bed shear stress of maerl from three contrasting hydrodynamic regimes, have been determined. In Chapter 2, the settling velocity of maerl has been experimentally measured and rigorously compared with theoretical models of settling velocity and detailed grain shape parameters. Quantitative modifications of the Ferguson and Church (2004) equation for settling velocity have been made by allowing the drag coefficient C2 parameter, which equates to the reciprocal of the convexity of the maerl grain, to vary with grain size. In Chapter 3, the critical bed shear stress of maerl is experimentally determined using three techniques; Law of the Wall, Turbulent Kinetic Energy and Reynolds Stress. The results show that maerl has a lower critical Shields parameter than quartz grains of an equivalent sieve diameter primarily due to their highly-irregular grain shape leading to greater drag experienced by the maerl grains and the relative grain protrusion. In Chapter 4, coupled hydrodynamic-wave-sediment transport models are computed using the DHI MIKE 21 suite of modelling tools and subsequently utilised to compute the spatially-varying tidally-induced sediment mobility and combined wave-current induced sediment mobility during calm and storm conditions. A grid of spatially-varying critical Shields parameter is computed for maerl areas and areas of Galway Bay with quartz siliciclastic sediments. Maerl is present at the periphery of wave-induced residual current gyres during storm conditions. The peak combined wave-current induced Mobilization Frequency Index during storm conditions is the key hydrodynamic parameter governing the distribution of maerl and siliciclastic sediments and is the most useful physical surrogate for maerl in predictive habitat suitability modelling studies. The thesis concludes by evaluating the utility of sediment mobility indices for marine spatial planning and for the design of Marine Protected Areas (MPAs).

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